The pinions with odd tooth counts are profile shifted. Within the same outer diameter, there is no need to adjust the backlash when changing the pinion.

Profile regraded:

This means that the number of teeth is changed while the diameter remained constant. The part of the involutes? curve that defines the tooth profile is changed. Positively regraded (lower number of teeth) pinions are stronger and run smoother, the reduced strength of negatively regraded pinions doesn´t really matter.